The app has been uploaded at https://cjhinolan.shinyapps.io/midpoint-rule/

Note:

1. Function can only be expressed in terms of x
2. Variables with coefficients have to be multiplied to be understood by the program ex: sin(2x) should be sin(2*x)
3. Due to the way the function line is graphed and presented in the web app, a visual bug may occur which renders the line off from the rectangle midpoints, especially at the beginning of the line. This does not affect the calculation of the program but it should be stated that the program may produce a slightly inaccurate graph at times.